Conventionally in press molding, the molded articles have corner portions, which are easy to form cracks or wrinkles, and the molded articles having the cracks or wrinkles are scrapped. So, before producing the press mold, the examination of the die-face design, the simulating evaluation and the like are carried out so as not to form the cracks or wrinkles.
However, after producing the press mold and performing the prototype molding, there are many cases that the examination and the like does not work.
Thus, some modifications are needed to produce the press mold that will not form the cracks and wrinkles, but the modifications cost much money and time.
The press molding using the conventional press mold is described below, referring FIGS. 4 to 9. In FIGS. 4, 8, 9, for convenience, a die 32 as an upper mold is not shown and they illustrate plan views seeing through the die 32.
Here, describing the terms used in below explanations, a drawing profile 21 illustrated by the broken lines in FIGS. 4, 5, is the border between a punch 33 as a lower mold and a blank holder 34 in plan view. Parting lines 22 illustrated by two-dotted lines are the borders between the product parts and the removed parts in the blank. That is, the part inner than the parting lines 22 is used as a product and that outer than the parting lines 22 is cut off.
In the area surrounded by the drawing profile 21 and an outline 23 of the blank, a clamping portion 37 is defined by a blank holding face 32a and a cushion face 34a. The clamping portion 37 holds the blank 35 with applying the inflow resistance to the blank 35 in the hatched area depicted in FIG. 4.
As shown in FIGS. 5, 6, a conventional press mold 31 has the die 32, the punch 33, and the blank holder 34.
In the pressing method called cushion drawing, FIG. 5(a) shows a first step mounting the blank 35 on the blank holder 34.
FIG. 5(b) shows a second step moving the die 32 downwardly and clamping a periphery 35a of the blank 35 by the clamping portion 37, that is the face 32a of the die 32, and the face 34a of the blank holder 34.
FIG. 6(a) shows a third step moved down the cushion face 34a in response to the pressure from the die 32, with holding the periphery 35a of the blank 35 by the clamping portion 37.
FIG. 6(b) shows a forth step pressing the blank 35 by a projection 33a of the punch 33, with the projection 33a facing a recess 32b of the die 32. The blank 35, clamped by the portion 37, flows into a cavity 36 defined by the projection 33a and the recess 32b in the arrow B1 and B2 directions, and plastically deforms along the shape of the projection 33a and the recess 32b. As shown in FIG. 7, the plastic deformation of the blank 35 is the curvature at the portions pointed by the numerals M1, M2 and the stretch by the length L. In this manner, the blank 35 is deformed to form the three-dimensional molded article.
As shown in FIGS. 4, 8, the clamping portion 37, which is configured by the faces 32a, 34a and clamps the periphery 35a of the blank 35, is formed in L shape to continuously surround a corner portion 35b of the blank.
In such a case, when the corner portion 35b is formed, as shown in FIG. 8, the inflow of the blank 35 is concentrated from the two sides (R side and S side depicted in FIG. 8) toward a corner recess 32c, as a result, the periphery 35a outer than the corner portion 35b (in detail, the area T in FIG. 8) is pressed to compress (“the contraction forming”).
Thus, as shown in FIG. 9, in the periphery 35a outer than the corner portion 35b (in detail, the area T in FIGS. 8, 9), the thickness increases.
The increase of the thickness in the periphery 35a (in the area T) results in the increase of the inflow resistance, so that the cracks and wrinkles are formed around the corner portion 35b (in detail, the area U in FIG. 9).
Further, when setting the curvature radius of the corner portion 35b as small value, the increase of the inflow resistance becomes high, and the curvature radius needs to be set as a proper value, which limits the designs.
To prevent the cracks and wrinkles at the corner portion of the molded article, the die is modified such that the blank holding face has a bead and the corner of the face has a projection and recess, whereby increasing the resistance against the wrinkles of the blank flowing into the corner portion, and preventing the cracks and wrinkles. For instance, Patent Literature 1 discloses such a technique.
Such a conventional technique unfortunately fails to prevent the cracks and wrinkles at the corner portion of the molded article if the drawing depth of the article is large.
[Patent Literature 1] JP H8-25097 A